1. Field of the Invention
The present invention relates to an apparatus and a method for irradiating a medium.
2. Description of the Related Art
Light scattering is one of the essential matters that can obstruct and even prevent viewing inside of, or through, a medium where scattering processes are dominant. This is because the scattered light does not propagate in a straight line through the medium, with the random paths of the scattered light causing the loss of directionality of the light as well as information associated therewith. Thus, it can be difficult to extract detailed internal information about a medium in which such scattering occurs via the detection of the scattered or diffused visible light. For example, in medical applications that deal with biological tissues, the scattering that occurs in passing light through the tissues may make it difficult to obtain internal information via detection of the scattered light.
In addition, there is also increasing demand to be able to concentrate light energy at a target position in a scattering medium, for example, to allow for treatment of abnormal tissue in photodynamic therapy, as well as to achieve unique and promising functions that were heretofore unobtainable in intentionally disordered random materials.
The ability to focus light at a point inside of or through a scattering medium has not been achieved until fairly recently. However, in recent years, a technique has been proposed which optimizes a wavefront of incident light to suppress the scattering effect.
In U.S. Patent Application Publication No. 2009/0009834 (Yaqoob et al., published Jan. 8, 2009), an optical phase conjugation technique is disclosed that can be used to record a wavefront of scattered light transmitted through a scattering medium by a holographic recording material, and to generate a phase conjugation wave, which has a phase substantially opposite to the phase of the recorded wavefront. The phase conjugation wave is generated such that it is configured to enter the scattering medium and to be viewable through the scattering medium.
Since elastic optical scattering is a deterministic and a time-reversible process, the optical phase conjugation can retrace its trajectory back through the scattering medium to its original incident point. The method as disclosed in U.S. Patent Application Publication No. 2009/0009834 utilizes this ability, which can be effective in suppressing the scattering effect and enhancing the spatial resolution of the images obtained of the scattering medium.
However, U.S. Patent Application Publication No. 2009/0009834 discloses that the optical phase conjugation method described therein is only capable of focusing light at a region just behind the scattering medium, where the incident light originally impinges. Therefore, as described therein, the method is not capable of focusing light arbitrarily at any specific point inside the scattering medium. However, U.S. Patent Application Publication No. 2009/0009834 further suggests that it may be possible to use the phase conjugation method described therein to illuminate some of the specific scatterers that cause strong forward scattering inside the medium.
This situation, however, is applicable only when the scattering property of the medium is so low that it can be assumed that scattering occurs only at a few specific points in the medium where strong scatterers are located. In addition, the specific points (locations) may need to be known to be able to take advantage of this focusing effect in further applications such as imaging or therapy.
In many cases, such as in a high scattering medium including biological tissues, this situation is not practical. For example, in a strong scattering medium, the multiple scattering processes that can occur therein may make it quite difficult to specify which scatterers are dominant and are mainly responsible for causing the forward scattering (and sometimes the scattering may not even be in the forward direction). It can thus be difficult to retrace and illuminate the light path created by the multiple scattering processes using optical phase conjugation. Such multiple scattering processes may be caused, for example, by the presence of too many of the scatterers in the medium.
Furthermore, it can be challenging to determine the exact location where the optical phase conjugation retraces to focus in such a scattering medium, because the location of the “strong scatterers” inside the medium are usually not known, even when the light is focused in a general vicinity of the “strong scatters”.
Thus, these points are of consideration in applying a phase conjugation method in a scattering medium.